(1) Field of the Invention:
This invention relates to a solid-state imaging device which has a plurality of picture elements disposed in a semiconductor surface region. More particularly, it relates to a solid-state imaging device which has picture elements for reading out from photodiodes photo information stored therein.
(2) Description of the Prior Art:
As a prior-art solid-state imaging device, there has been one as shown in FIG. 1.
FIG. 1 shows an example of construction of a typical areal solid-state imaging device. In this device, a photodiode 1 and an insulated-gate field effect transistor (shortly termed "MOS Tr") 2 make up a picture element as a unit. The array of the picture elements is selected by a horizontal scanning circuit (horizontal scanner) 9 and a vertical scanning circuit (vertical scanner) 10 which are made of MOS Tr type shift registers. Thus, MOS Tr's 3 and the MOS Tr's 2 are respectively turned "on" and scanned in sequence. By the scanning, charges having been generated by light as stored in the photodiodes 1 are led out from an output terminal 8 through signal lines 6 and 7, and video signals which the picture elements have received are taken out as electric signals. In FIG. 1, numeral 5 indicates a video power source. For some purposes, a plurality of signal lines 7 and output terminals 8 are disposed (for example, in case where the device is used in a color TV camera, the picture elements are allotted to respective color lights of the three primary colors, and signal lead-out lines for the respective primary color lights are disposed).
FIG. 2 shows the sectional structure of a typical picture element. Hereunder, for the sake of convenience of the description, an N-channel type solid-state imaging device in which the signal charges are electrons will be stated. However, the following explanation is quite similarly applicable to a P-channel type device by merely inverting the conductivity type and the polarity.
The pn-junction photodiode is formed of a silicon (Si) substrate 11 which is made of monocrystalline silicon of the p-type conductivity, and a diffusion layer of the n-type conductivity (n-type diffused layer) 12 which serves as a storage region for signal charges. The n-type diffused layer 12 simultaneously serves as a source, to form the insulated-gate field effect transistor (MOS Tr) serving as signal charge detection means along with a gate electrode 13 made of, e.g., polycrystalline silicon, a silicon dioxide (SiO.sub.2) film 16 being thin under the gate electrode 13 and an n-type diffused layer 14 serving as a drain. The n-type diffused layer 14 is usually provided with an electrode 17 of a metal such as aluminum (Al) in order to lower the electric resistance thereof, and the electrode 17 is used as the signal line 6 in FIG. 1. The SiO.sub.2 film 16 is ordinarily thickened outside the picture element in order to suppress the generation of an unnecessary stray capacitance.
Upon incidence of light 15, electron-hole pairs are generated in the n-type diffused layer 12 and the Si substrate 11. The electrons in the pairs flow into the n-type diffused layer 12 as the signal charges, and are stored in the pn-junction capacitance between the n-type diffused layer 12 and the Si substrate 11. The horizontal switching MOS Tr 3 is rendered conductive by a positive scan pulse from the horizontal scanner 9, and through the signal line 6 connected thereto, a video voltage from the video power source 5 is applied to the drains of the MOS Tr's 2 connected to the signal line 6. When a positive scan pulse from the vertical scanner 10 is simultaneously impressed on the gate electrode 13 of the MOS Tr 2, the signal charges (electrons) are drawn by and to the n-type diffused layer (drain) 14 held in a positive potential and are led to the output terminal 8 through the drain electrode 17.
The potential of the n-type diffused layer 12 consequently becomes a positive potential, which drops because the pn-junction capacitance continues to store electrons developed by the light 15 until the next positive scan pulse is impressed. Since the quantity of electrons stored corresponds to the quantity of light in the image of an object projected on the pn-junction photodiodes of the respective picture elements, the video signal can be taken out by the above operation.
Such a prior-art solid-state imaging device, especially the pn-junction photodiode being the heart thereof, structurally involves the disadvantage that the sensitivity to the visible light, especially short wavelength light, being the object of the image pickup is low. Accordingly, notwithstanding that a practical solid-state imaging device is eagerly desired, it is not yet realized.
The absorption of light by silicon differs depending on the wavelength. The absorption characteristics for lights of red (wavelength: 0.65 .mu.m), green (0.55 .mu.m) and blue (0.45 .mu.m) constituting the three primary colors are illustrated as R, G and B in FIG. 3, respectively. The blue light at B creates electron-hole pairs in the vicinity of the surface of silicon, while the red light at R penetrates deep into silicon and creates electronhole pairs. In the n-type diffused layer near the surface of silicon, the probability at which the electron-hole pairs disappear due to the recombination is high, and the sensitivity lowers to that extent. As the result, the pn-junction photodiode employing silicon becomes highly sensitive to the red light (light of longer wavelength). According to a measurement by the inventor, even in case where the junction depth of the pn-junction photodiode is made as small as below 0.8 .mu.m in order to enhance the sensitivity to the blue light, the sensitivity in blue is lower than the sensitivity for red.
In case of the color image pickup, accordingly, there occur such severe problems that the fidelity of color reproduction to the blue light becomes extremely inferior and that the signal-to-noise ratio is low. In effect, the color image pickup is impossible. Even in the black-and-white image pickup, a blue part in the image of an object becomes blackish, and a red part becomes whitish, so that the reproduced image becomes an unnatural one widely different from the image of the object. These drawbacks are great hindrances to the realization of the practical solid-state imaging device.